This invention relates to telephone line interface circuits, and is particularly concerned with a method of operating a telephone line interface circuit to avoid creating potentially hazardous conditions on a telephone line to which the line interface circuit is connected in use.
Various methods of applying high voltage signalling to a telephone subscriber line are well known. The most common example of high voltage signalling is the telephone ringing signal, and accordingly this is referred to predominantly below, but other forms of high voltage signalling, for example coin signalling and message indicating, are also well known.
A ringing signal generator originally typically consisted of rotating machinery provided commonly for many telephone lines, the machinery including a commutator for distributing bursts of a ringing signal to various lines. Similar generator arrangements existed for generating other high voltage signals. Such generators were considered acceptably safe in that they could not create hazardous voltage and current conditions on a telephone line; if the commutation ceased due to a failure of the machinery to rotate, the generation of voltage and current also ceased. Thus for example such a generator could not generate a high d.c. voltage.
More recent telephone systems have generally continued the practice of generating high voltage signalling in a common part of the system. Using a common generator, it has been feasible to provide relatively elaborate measures to avoid the possibility of creating hazardous voltage and current conditions on the telephone lines. Such hazard prevention measures may be implemented using dedicated hardware circuitry, with duplicated or redundant systems for increased reliability.
The parent and cross-referenced applications above relate to a programmable telephone line interface circuit which includes an individual software-controlled voltage generator, of the form described in U.S. Pat. No. 5,103,387, which is necessarily designed to be capable of generating relatively high voltages and currents. While these voltages and currents are considered to be safe within certain normal operating conditions typically imposed by telephone system administrations, faults can arise which could cause the controlled voltage generator to operate beyond these conditions, with the possibility of creating potentially hazardous voltage and current conditions on the telephone line.
For example, in order to produce a ringing voltage waveform, the software in the line interface circuit supplies a low voltage reference waveform to the controlled voltage generator, whose output is a corresponding waveform at high voltage which is supplied to the telephone line via relay contacts or switches. A software fault could cause samples of the reference waveform not to be updated in their normal sequence, possibly creating a high d.c. voltage produced by the controlled voltage generator and supplied to the telephone line. Conversely, a hardware fault such as a stuck relay contact could cause an undesired high voltage to be produced by the controlled voltage generator and supplied to the telephone line. In either case, a potentially hazardous, and therefore impermissible, voltage would be applied to the telephone line.
Various other possible fault situations, some of which are discussed further below, can be envisaged in which there is a potential for a hazardous voltage or current condition to be produced on the telephone line. This potential is not diminished by the nature of the line interface circuit, specifically that it must be produced in large numbers at low costs and must provide consistently reliable service over a period of many years. To facilitate such service it is also desirable to avoid faults giving rise to voltage or current conditions which may be potentially damaging to the line interface circuit itself.
The relatively elaborate measures to avoid hazardous conditions, known for high voltage generators which are common to many telephone lines, are impractical and not economically feasible for high voltage generators provided on individual line interface circuits. For example, the costs and space requirements of dedicated hardware circuitry and duplicated systems would be prohibitive for individual line interface circuits.
An object of this invention, therefore, is to provide a method of operating a telephone line interface circuit, including a generator for high voltage signalling, in which the potential for creating a hazardous condition on the telephone line in the event of a fault is substantially reduced or eliminated.